1. Field of the Invention
This invention relates to a white color light emitting diode (LED) which can produce white light by a single LED chip and further relates to a neutral color LED which can make redpurple light or pink light which is a neutral color between red and blue by a single LED. In particular, this invention is directed to the structure of the white color LED and the neutral color LED. White light is an assembly of a plurality of wavelengths including blue, red, green or so. There is a strong desire for a new light source of white. White light is most suitable for illuminating light sources, since white light includes all primary colors. White light is appropriate for various displays. White light is also used for the backlight of liquid crystal displays (LCD). Neutral tint LEDs between red and purple are also suitable for displays and illumination. This invention proposes the neutral color LED and the white color LED suitable for illumination, displays, LCD backlight and so forth.
This application claims the priority of Japanese Patent Application No. 10-194156(194156/1998) filed on Jul. 9, 1998, Japanese Patent Application No. 10-316169(316169/1998) filed on Nov. 6, 1998 and Japanese Patent Application No. 10-321605(321605/1998) filed on Nov. 12, 1998 which are incorporated herein by reference.
2. Description of Related Art
An LED produces light by lifting electrons by a current and throwing down the electrons over the band gap (forbidden band) between a valence band and a conduction band. The electron transition energy generates light. The band gap is equal to the energy of a photon which is a quantum of light. The band gap of an active layer gives the wavelength of the emitting light. The wavelength determines the color of the light. The color of the light depends upon the material of the active layer of an LED.
All the conventional LEDs have utilized only the electron band gap transition for making light. All the band gap transition LEDs emit monochromatic light (monochromatic LEDs). Monochromatic LEDs of emitting red, yellow, green or blue color have been produced and sold. For example, red light high luminescent LEDs which produce stronger power than several candelas (Cd) have been put on sale. The red light LEDs are based upon active layers of aluminum gallium arsenide (AlGaAs) or gallium arsenide phosphide (GaAsP). Inexpensive red light LEDs have wide scopes of applications. Green/yellowgreen light LEDs having a gallium phosphide (GaP) light emitting layer (active layer) have been manufactured and sold, too. Blue light LEDs including an SiC layer as an active layer have been proposed. Blue/green light LEDs based on an active layer of gallium indium nitride (GaInN) have been on market. LEDs having an AlGaInP active layer are orange/yellow color LEDs. Monochromatic LEDs having the following combinations of the colors and the active layers have been manufactured.
These are already matured as inexpensive practical LEDs. Among these LEDs, GaP LEDs and SiC LEDs have not attained to higher power emission than one candela, because GaP and SiC are indirect transition type semiconductors. What determines the wavelength is the material of the active layer. Such a crystal, that has a desired band gap and satisfies conditions, for example, the lattice matching condition and so on, is selected as an active layer.
All the conventional LEDs can make a single color, because the LEDs make use of the photon emission induced by the band gap transition of electrons. Thus, the conventional LEDs are all monochromatic light sources. Monochromatic LEDs have wide scope of utility for displaying light sources. However, monochromatic LEDs cannot replace all the current light sources. Monochromatic light is impotent to use lighting (illumination), special displays or LCD backlight, since the monochromatic light includes only the light having a single wavelength. If a monochromatic LED were used for a lighting source, illuminated objects would all wear the color emitted from the monochromatic LED instead of the inherent color of the objects. If a monochromatic LED were employed for LCD backlight, the LCD would show monochromatic images of the color.
Lighting or illuminating requires white color light sources which inherently include all primary colors and neural color light sources which include neutral colors between purple and red. However, there have been no semiconductor LEDs capable of emitting white light yet. Illuminating light is still supplied by incandescent light bulbs or fluorescent lamps in general. Although being cheap, incandescent light bulbs are suffering from a short lifetime and a low luminous efficiency. Fluorescent lamps also suffer from a short lifetime, though they enjoy a higher luminous efficiency than the incandescent light bulbs. Further, the fluorescent lamp requires heavy accessories, e.g. voltage stabilizers. The fluorescent lamps have further the drawbacks of a big size and a heavy weight.
It is hoped that future white and red-purple neutral color light sources satisfy the attributions, that is, small size, simple accessories, long lifetime, high luminous efficiency and low price. One candidate capable of sufficing these difficult requirements would be a semiconductor light emitting device (LED or LD). LEDs are small, light and inexpensive light sources having a long lifetime and high efficiency. However, since LEDs utilize electron transitions across the forbidden gap between the valence band and the conduction band, the LEDs inherently emit monochromatic light. Neither single LEDs nor single LDs can generate white color light due to the electron band gap transition emission. Monochromaticity is the inherent property of LEDs.
With regard to neutral colors, the conventional LEDs can make primary colors (RGB) and restricted neutral colors. The colors the current LEDs can produce are red, orange, yellow, yellowgreen, green, bluegreen, blue, bluepurple and purple. Among them, red, green and blue are primary colors. Orange, yellow and yellowgreen are neural colors between red and green. Bluegreen, bluepurple and purple are neutral colors between blue and green. Among three primary colors, red has the longest wavelength, green has a middle wavelength and blue has the shortest wavelength. Blue and green are a nearer pair. Green and red are another nearer pair. LEDs can make neutral colors between two neighboring primary colors (R-G and G-B). Any neutral colors of the conventional LEDs are still monochromatic colors which possess only one wavelength. Conventional LEDs basing on the band gap transition can produce monochromatic R-G or G-B neutral colors.
Any conventional LED can make neither neutral color between red and blue (R-B) nor neutral color among red, green and blue (R-G-B). Red and blue have very different wavelengths. Neutral colors between blue and red (B-R) and among blue, red and green (P-G-B) are no more monochromatic colors having a single wavelength but complex colors including a plurality of wavelengths. Thus, the white (R-G-B) color and the R-B neutral colors cannot be produced by the electron band gap transition in principle.
Instead of monochromatic light sources, lighting, ornament or display requires neutral colors being a mixture of red and blue and white color being a mixture of blue, green and red. Conventional LEDs utilizing the electron band gap transition are all monochromatic light sources. Although the conventional bluegreen LEDs, bluepurple LEDs and orange LEDs are essentially monochromatic LEDs, each LED has only one peak of wavelength somewhere in the emission spectrum.
The neutral color in the present invention does not mean a monochromatic color intervening between two primary colors but a mixture of primary colors. Monochromatic light source has a single peak in the spectrum. But, the neutral color of the present invention has at least two peaks in the spectrum.
A white color LED would probably be produced by assembling a red color LED, a green color LED and a blue color LED. Red color LEDs and green color LEDs have been widely produced and sold on the market. It had been harder to make blue color LEDs than red or green color LEDs due to the difficulty of making good crystals having a wide band gap. Recently, the blue color LEDs based upon a GaInN active layer and a sapphire substrate have been invented. The blue color LEDs are fabricated and placed on the market at the present time. Three primary color (green, red and blue) LEDs are already on sale. A white color LED could be made by assembling a red color LED, a green color LED and a blue color LED. However, three-component LEDs would enhance the cost of the hybrid LED. The three LEDs would consume electric power three times as large as a single LED. The three-component LED would require a sophisticated power balance for making the white color suitable for illuminating or lighting. The necessity of regulating the power balance would complicate a driving electric circuit. The assembly of the component LEDs would enlarge the device size. Three-component white color LED would bring about no advantages over prevalent incandescent bulbs or fluorescent lamps. It is preferable to make white light by a single LED instead of a set of three LEDs.
A trial has been proposed for fabricating a white color LED consisting of a GaN-type blue LED and a YAG phosphor layer. The YAG-GaN LED is a first-proposed white color LED, which is described in the following textbook. {circle around (2)} Shuji Nakamura and Gerhard Fasol, xe2x80x9cThe Blue Laser Diode (GaN Based Light Emitters and Lasers), January 1997, Springer, p216-221(1997)xe2x80x9d.
The YAG-GaN LED is made by burying a GaN-type blue light LED having a GaInN active layer into a YAG pond emitting yellow fluorescence. YAG is an abbreviation of yttrium aluminum garnet. FIG. 1(a) shows a section (a) of the proposed GaN-YAG LED, and FIG. 1(b) shows an enlarged section of the proposed GaN-YAG LED. A dome-shaped transparent plastic mold 1 holds a first stem 2 and a second stem 3. The xcex93-formed first stem 2 has a top side arm with a small cavity 4. A GaN-type blue light LED chip 5 having a GaInN active layer is set on the bottom of the cavity 4. The LED 5 has cathode and anode electrodes on the top. The electrodes are connected to the stems 2 and 3 by wires 7 and 8. The cavity 4 is filled with a yellow YAG phosphor 6 for fully covering the GaN-type LED 5. After hardening of the YAG resin, the stems 2 and 3 are molded by the transparent plastic 1.
Conventional photodiodes (PDs) and light emitting diodes (LEDs) are used to employ conductive substrates. Such a conductive substrate can be one of electrodes, mainly a cathode. The conventional PD or LED has only a single electrode (mainly anode) on its top which is connected to a stem by a single wire. However, the current GaN blue light LED employs an insulating sapphire (Al2O3) crystal as a substrate due to the difficulty of growing good GaN single crystals. A GaN layer is grown on the sapphire substrate and a GaInN active layer is piled on the GaN layer in the GaN-type LED 5. The insulating sapphire substrate cannot be a cathode. The cathode is formed side by side with the anode on the top of the chip. The two top electrodes require two wires for connecting with the stems. The GaN-type blue LED 5 emits blue light when a current flows from the anode to the cathode. A part of the blue light passes through the YAG phosphor pond 6 to an external space. The rest of the blue light is absorbed by the YAG phosphor pond 6 and is converted to yellow light having a longer wavelength than the parent blue light. The YAG pond 6 emits yellow converted light (F). The LED 5 emits blue light (E). The yellow light (F) and the blue light (E) together go out of the plastic mold 1. The yellow and the blue are naturally synthesized. The synthesized color is white, when the ratio of the blue light power and the yellow light power lies within a suitable scope. The GaN-YAG aims at making white color light by superposing the blue light of the GaN-LED on the yellow fluorescence from the YAG pond excited by the blue light.
The LED positively produces light by lifting up and down electrons across the band gap (forbidden band) between the conduction band and the valence band. The fluorescent material passively makes light. When the fluorescent material absorbs the LED light, some electrons jump from the ground band to an excited band. The electrons stay at the excited band for a short time and fall back to the ground band via extra levels called xe2x80x9cfluorescence centersxe2x80x9d. The fluorescence produces the light of lower energy than the original LED light. When the LED is enclosed by the fluorescent material, the LED emits the inherent blue light, and the fluorescent material emits fluorescent light having a longer wavelength than the inherent blue light. The YAG generates yellow fluorescence excited by blue light. When the blue light and the yellow fluorescence mix together in a proper ratio, white color light is synthesized. Blue has the shortest wavelength and the highest energy among three primary colors. The appearance of a blue light LED enables to produce white light.
FIG. 2 shows the emission spectrum of the YAG-GaN LED. The abscissa is the wavelength(nm). The ordinate is the emission intensity (arbitrary unit). The sharp peak of 460 nm originates from the GaN-type LED. 460 nm is equal to the band gap of GaInN. The broad peak of 550 nm arises from the fluorescent YAG pond. Human eyesight cannot discriminate the components (460 nm and 550 nm) of light. The synthesized light seems white color.
The proposed YAG-GaN LED, however, has some drawbacks, which will be pointed out as follows.
(1) Translucent YAG phosphor is filled in the cavity covering the LED. The YAG absorbs the LED light, which brings about a low external quantum efficiency. Although a strong GaInN LED having inherently more than 1 candela of luminosity and more than 5% of external quantum efficiency is employed, the white YAG-GaInN LED has only 0.5 candela and 3.5% of external quantum efficiency due to the absorption by the YAG. The poor transparency of YAG decreases both the luminosity and the quantum efficiency.
(2) The conversion efficiency of the YAG phosphor is only 10%. Such a low conversion efficiency decreases the yellow component. If the thickness of the YAG were increased for reinforcing yellow, the luminosity would be further decreased by the thick YAG. The external quantum efficiency would further be reduced.
(3) The YAG-GaInN hybrid LED requires the YAG phosphor which is an entirely different material from GaN. The existence of a foreign material increases the steps of production. The process cost would be pushed up.
(4) Since the YAG phosphor is filled in the cavity and covers the GaInN-LED. The YAG raises the material cost. The complex shape of the stem for the YAG pond enhances the cost of the stem.
LEDs generally enjoy advantages of small-size, inexpensiveness, low-current and long lifetime. One purpose of the present invention is to provide a white color LED for emitting white color which is an assembly of red, green and blue. Another purpose of the present invention is to provide a neutral color LED emitting red-blue neutral colors e.g., redpurple, pink or so.
As shown in FIG. 30, this invention tries to propose a white color LED which synthesizes white color {circle around (1)} and neutral color LEDs which produce purple{circle around (2)}, redpurple{circle around (3)} purplish pink {circle around (4)}, pink {circle around (5)} and yellowish pink {circle around (6)}.
FIG. 31 is a general chromaticity diagram. The chromaticity diagram is a graph showing the two-dimensional coordinates of a visible light source color or a visible object color by dividing and numerizing the color stimulus into the stimuli of primary colors red(R), green(G) and blue(B) which correspond to three kinds RGB of color-sensing organs in a human eye. Q(xcex) denotes the spectrum of a light source. The RGB stimuli on the color-sensing organ are obtained by multiplying the object spectrum Q(xcex) by the color matching functions for the primary colors RGB. Here, r(xcex) is the red color matching function, g(xcex) is the green color matching function and b(xcex) is the blue color matching function. The red stimulus X to the color-sensing organ is given by X=∫Q(xcex)r(xcex)dxcex. The green stimulus Y to the human sensing organ is Y=∫Q(xcex)g(xcex)dxcex. The blue stimulus Z is Z=∫Q(xcex)b(xcex)dxcex. The chromaticity diagram is a set (x,y) of a normalized red stimulus x and a normalized green stimulus y. The normalized red stimulus x and green stimulus y are given by summing the three integrated stimuli X, Y and Z to (X+Y+Z), dividing the red stimulus X and the green stimulus Y by the sum (X+Y+Z) and obtaining x=X/(X+Y+Z) and y=Y/(X+Y+Z). The normalized z=Z/(X+Y+Z) will be omitted from now for reducing the number of the chromatic parameters. The normalized blue stimulus z can be easily obtained from x and y, since x+y+z=1. The coordinate (x,y) is the set of normalized red stimulus and the normalized green stimulus in the chromaticity diagram. The coordinate system can denote any color by a single point lying within the rectangle isosceles triangle with three corners (0,0),(1,0) and (0,1).
The boundary solid line of a horseshoe shape denotes monochromatic colors in FIG. 31. The horseshoe-shaped boundary curve is determined by the three color matching functions r(xcex), g(xcex) and b(xcex). For example, in the range of the wavelengths of longer than 550 nm, the sensitivity for blue is zero (z=0), the chromaticity coordinates (x,y) of monochromatic colors lie on the line x+y=1. In the ranges of wavelengths shorter than 505 nm, a decrease of the wavelength increases the blue component accompanied by a slow rise of the red component, which separates the monochromatic curve from the y-axis (x=0). Red end of the horseshoe-shaped monochromatic curve is the longest wavelength limit of 680 nm to 980 nm of the visible light. Blue end of the horseshoe curve is the shortest wavelength limit of 380 nm to 410 nm of the visible light. The shortest wavelength end and the longest wavelength end are connected by a straight line which does not correspond to monochromatic colors at all. The straight line is called a purple boundary. The inner region enclosed by the horseshoe curve and the purple boundary denotes neutral colors. The innermost region is the white color region. As shown in FIG. 31, the white region ranges from x=0.22 to x=0.43 and from y=0.21 to y=0.43. Conventional LEDs could not produce the white light within a single device. The lower regions of the neutral colors of pink, purple, redpurple can not be made by the conventional LEDs. One purpose of the present invention is to provide a white color LED which emits white light {circle around (1)} in FIG. 30. Another purpose of the present invention is to provide neutral color LEDs which can produce neutral colors {circle around (2)}, {circle around (3)}, {circle around (4)}, {circle around (5)} and {circle around (6)} below white color in FIG. 30.
Instead of adding a phosphor to an LED, this invention makes the best use of the substrate itself as a fluorescent source. This invention gives the substrate the new role of the fluorescence source which absorbs the LED light from the active layer and produces the light of a longer wavelength than the LED light. An LED has a substrate on which an active layer or other layers are deposited. The active layer positively produces the light of a wavelength determined by the band gap. In the conventional LEDs, the substrate has no contribution to making light. What was the role of the substrates in prior LEDs? The substrates have had only two passive roles of supporting the epitaxial light emission structure and of leading driving current so far.
This invention makes the best use of the ZnSe substrate as a fluorescent material by doping some impurity into the ZnSe substrate. The ZnSe-type active layer emits blue light of a shorter wavelength. The fluorescent substrate produces yellow or orange light of a longer wavelength. White color or neutral colors are made by synthesizing the blue light from the ZnSe-type active layer and the yellow or orange fluorescence from the ZnSe substrate. The advantage of the present invention is to convert a blue light LED to a white or neutral color LED without adding any new parts.
The LED of the invention makes white and neutral color light by combining the active layer emission and the substrate fluorescence. In the case of a single photon absorption process, the fluorescence has a longer wavelength than the original exciting light. Thus, blue light with a short wavelength is pertinent to the excitation light. If the excitation light were longer wavelength light, the synthesized light would be neither white light nor RB neutral light. The excitation light should be blue or bluegreen. Blue excitation light restricts the kinds of the active layers which produce the excitation light by the electron band gap transition. The active layer must have a band gap energy corresponding to the blue. GaInN-type activation layers and ZnSe-type activation layers are known as blue light sources. This invention prefers ZnSe-type active layers for the excitation light sources. The substrate should be fitted to the active layer. The restriction of the lattice matching determines the material of the substrate. Since the active layer has been determined to be ZnSe, the preferable substrate should be ZnSe from the lattice matching condition. Of course, ZnSe-type LEDs have been already produced as blue light LEDs till now. But most of the ZnSe-LEDs have been made upon GaAs substrates (ZnSe/GaAs), since GaAs wafers with low defect density can be easily fabricated and GaAs satisfies the lattice-matching condition to ZnSe. A few of the ZnSe-LEDs have semi-insulating ZnSe substrates. The insulating ZnSe substrate requires two wires for coupling the n-electrode and p-electrode to two leads. None of the blue ZnSe-LEDs have conductive ZnSe substrates.
This invention adopts conductive ZnSe as a substrate for the ZnSe active layer for satisfying the lattice-matching condition. The ZnSe substrates are suitable for other reasons in this invention. We found that some ZnSe substrates have a character of fluorescence.
When ZnSe is doped with iodine (I), aluminum (Al), chlorine (Cl), bromine (Br), gallium (Ga) or indium (In), the ZnSe is converted into an n-type semiconductor. The n-type conduction reduces the resistivity of the ZnSe substrate. At the same time, the impurity atoms form emission centers in the ZnSe substrate. The emission centers absorb short wavelength light, and convert the light to longer wavelength light and emit the longer wavelength light. Absorbing the light of a wavelength shorter than 510 nm, the impurity centers emit self-activated luminescence (SA emission) having a broad spectrum of wavelength ranging from 550 nm to 650 nm. The emission is called self-activated emission. The emission center is called an SA center. The middle wavelength and the full width at half maximum (FWHM) of the SA emission spectrum can be controlled by the selection of impurities (I, Al, Cl, Br, Ga and In) and the concentration of the impurity. The SA emission spectrum is widely dispersed between red and yellow.
In general, ZnSe-type active layers can produce blue light of a wavelength shorter than 510 nm. ZnSe substrates can absorb the light of a wavelength shorter than 510 nm which is longer than the band gap wavelength (460 nm) due to the band tailing phenomenon and can produce SA-emission. The band tailing phenomenon which is important for this invention is inherent and peculiar to impurity-doped ZnSe. Thus, the band tailing phenomenon is clarified. An ordinary semiconductor without defects cannot absorb the light of a wavelength longer than the band gap wavelength. Namely, the ordinary semiconductor does not absorb the light of a wavelength xcex greater than xcexg (=hc/Eg). When the semiconductor includes impurities which form impurity levels in the forbidden band, the impurity levels induce extra transition between the impurity levels and the conduction band or between the valence band and the impurity levels. The substrate can absorb the light of a wavelength xcex greater than xcexg due to the impurity levels. The substrate can also the band gap transition emission xcexg of course. Then, the substrate becomes opaque to the light emitted from an active layer having the same component as the substrate. This is the band tailing phenomenon. What produces the impurity levels is aluminum, iodine, bromine or chlorine, which is called now SA-centers.
The device of the present invention includes two components of;
(1) ZnSe-type LED which emits blue light (460 nm to 510 nm) by the band gap transition of electrons, and
(2) ZnSe substrate which emits self-activated light between yellow and red (550 nm to 650 nm).
The excellence of the present invention is a simple structure. In general, an LED is produced by growing various epitaxial layers including an active layer on a substrate. The substrate is indispensable to the LED. The substrate of a prior LED plays only the roles of supporting epitaxial layers and leading an electric current to the active layer. The substrate is a passive component in the prior LED. This invention, however, makes the best use of the substrate as a light emission component. The white light LED is produced by doping an impurity into the ZnSe substrate of a ZnSe blue light LED. This invention increases one step of fabrication but adds no component to the ZnSe LED.
An assembly of the ZnSe active layer and the ZnSe substrate brings about an LED which produces both the blue light and the yellow light. The present invention takes advantage of the properties of the impurity-doped ZnSe substrate causing the SA-emission and the active layer yielding blue light.
The dopants (I, Al, Cl, Br, Ga, In) gives the n-type conduction to the ZnSe substrate in addition to the SA-centers. An LED can be produced by epitaxially growing an n-buffer layer, an n-cladding layer, an active layer and p-contact layer on the n-ZnSe substrate. Both the n-cladding layer and the p-cladding layer have refractive indexes lower than the active layer and band gaps larger than the active layer. The lower refractive indexes and the larger band gaps of the cladding layers have an action of enclosing the carriers and photons within the active layer. The set of the active layer, the contact layer and the cladding layers is called an xe2x80x9cepitaxial emission structurexe2x80x9d in the present invention.
The epitaxial emission structure includes one of the following active layers;
(1) ZnSe
(2) ZnCdSe
(3) ZnSeTe.
The epitaxial emission structure can produce blue (LED) light ranging from 460 nm to 510 nm according to the band gap energy. Since all the blue (LED) light has a wavelength less than 510 nm, the active layer light can induce SA-emission at the SA-centers in the ZnSe substrate through the band tailing phenomenon. Consisting of ZnSe, active layer (1) emits the light of a wavelength of 460 nm to 465 nm. Being a mixture of ZnSe and CdSe, active layer (2) makes the light of a wavelength longer than (1). The mixture ratio x (Zn1xe2x88x92xCdxSe) is omitted here. Active layer (3), a mixture of ZnSe and ZnTe, also makes the light of a longer wavelength than (1). Different mixture ratios x give the active layers (2) and (3) various light of wavelengths between 460 nm and 510 nm.
The LED has an n-ZnSe substrate and a set of epitaxial films of an n-cladding layer, an active layer, p-cladding layer and a p-contact layer grown on the n-ZnSe substrate. A p-electrode is formed upon the p-contact layer. An n-electrode can be formed on the bottom surface of the ZnSe substrate, since the substrate is endowed with the n-type conductivity. Both the p-side and the n-side can be assigned to an outlet of the light. In the case of the p-side outlet, the p-electrode should be a small dotted electrode, an annular electrode having a central opening or a transparent electrode. In the case of the n-side outlet, the n-electrode should be a small dot electrode, an annular electrode or a transparent electrode. In any cases, the counter electrode can be a wide electrode covering the overall surface for die-bonding directly on the stem.
When an electric current is supplied to the LED for flowing the current across the pn-junction, the active layer makes blue light according to the band gap energy. The dopants of the substrate absorb a part of the blue light and induce the SA-emission of yellow or orange. White color light or neutral color light is produced by the mixing of the active layer blue light and the substrate yellow and orange light.
xe2x80x9cWhite colorxe2x80x9d includes a wide range of various tones because white is a collective concept. If blue color prevails, the white tends to xe2x80x9ccold whitexe2x80x9d. On the contrary, if yellow is prevailing, the white trends to xe2x80x9cwarm whitexe2x80x9d. The thicker the ZnSe substrate is, the more the blue light emitted from the LED part is absorbed. When the yellow SA-emission is prevailing, the white trends toward xe2x80x9cwarm whitexe2x80x9d. On the contrary, a thinner ZnSe substrate makes colder white by decreasing both the absorption of blue light and the SA-emission of yellow light. The intensity of the SA-emission can be controlled by varying the thickness of the ZnSe substrate. Namely, the thickness of the ZnSe substrate is an important parameter for determining the ratio of the SA-emission to the LED band gap emission. However, the scope of the substrate thickness is restricted by other conditions. Less than 10 xcexcm of substrate thicknesses would increase the probability of breaking the substrate at the following steps and would decrease the yield which results in the enhancement of production cost. More than 2 mm of substrate thicknesses would cause too bulky LEDs and would enhance the ratio of the yellow SA-emission beyond white color. The preferable range of the ZnSe substrate thickness is 10 xcexcm to 2 mm.
The middle wavelength of the SA-emission spectrum can be varied by the choice of dopant and the concentration of the dopant (impurity), as mentioned before. The ratio of the SA-emission can be adjusted by the thickness of the substrate. Various tones ranging from warm white to cold white can be obtained by regulating the three important parameters, the dopant selection, the dopant concentration and the substrate thickness.
A concept of the present invention is shown in FIG. 3(a) and FIG. 3(b). FIG. 3(a) is a section of the whole LED of the invention. FIG. 3(b) is a section of a part of the LED. A xcex93-shaped stem 12 has a ZnSe-type LED 15 on the top in a plastic package 11. Another stem 13 dangles from the package 11 in parallel with the stem 12. As shown in FIG. 3(b), the LED 15 has a ZnSe substrate 16 and an epitaxial emission structure 17 grown on the substrate 16. The ZnSe substrate is doped with iodine (I), chlorine (Cl), bromine (Br), aluminum (Al), indium (In) or gallium (Ga) as SA-centers. The bottom surface of the ZnSe substrate 16 is an n-electrode directly bonded on the stem 12 as a cathode. The epitaxial film emission structure 17 includes an n-cladding layer, active layer, p-cladding layer and p-contacting layer. An annular or dot n-electrode is formed on the p-contact layer. The p-electrode is connected by an wire 18 to the stem 13 as an anode.
A driving current induces active blue light (B) in the epitaxial film structure 17. A part of the blue light goes upward out of the LED as blue light. The rest goes downward into the substrate 16 and induces yellow-orange luminescence (Y) at the SA-centers of I, Cl, Br, Al, In or Ga atoms. The SA emission goes upward. The light going upward out of the LED is a sum of the blue light (B) and the SA emission (Y). The synthesized light is white color or neutral colors. The neutral colors are, for example, pink, purple or redpurple lying between red and blue in the chromaticity diagram.
There are some alternatives for the geometric arrangements of the white color LED. One choice is the ordinary arrangement setting the substrate down and the epitaxial film part up like common LEDs. Another choice is the up-side down arrangement positioning the substrate up and the epitaxial film part down. A further contrivance is directed to the structures of the package and the stems for preventing only blue light from going out in a certain direction.
[TYPES OF WHITE OR NEUTRAL COLOR LEDS]
(1) Normal posture type ZnSe white color LED and ZnSe neutral color LED
FIG. 4(a) and FIG. 4(b) show an example of a white color LED or a neutral color LED of the present invention. FIG. 4(a) is a vertical section of the LED device. FIG. 4(b) is a vertical section of only the LED chip. A transparent plastic mold 11 encloses a stem 12, a stem 13 and an LED chip 15. The structure is similar to the conventional LED. The transparent mold package is the cheapest and the common package for LEDs. Of course, a metal can-type package is also available for the LED of the present invention. The kinds of stems and packages can be freely chosen in accordance with purposes. The xcex93-shaped stem 12 has no cavity on the top branch 14. The top branch 14 has an even plane. The ZnSe LED 15 of the present invention is fixed on the even plane 14 in the normal posture having a bottom ZnSe substrate 16 and a top epitaxial film 17. The ZnSe substrate 16 is doped with a dopant as SA-center, and the film emission structure, that is, the epitaxial film 17, is epitaxially grown on the ZnSe substrate 16.
The epitaxial emission structure 17 contains films of ZnSe or ZnCdSe and a pn-junction. In general, the epitaxial emission structure 17 is a set of strata of films containing ZnSe as a main component. A ring-shaped or dot-shaped p-electrode is formed on the top region above the pn-junction. The p-electrode is connected to the stem 13 by a wire 18. An n-electrode on the bottom of the substrate is directly connected to the stem 12. One wire is sufficient to connect the LED to the stems, unlike the GaN-YAG LED of FIG. 1(a). The stem 12 is a cathode, and the stem 13 is an anode. A current flowing the pn-junction induces the electron transition over the band gap and produces the light (E) of a wavelength between 460 nm and 510 nm. A part of the intrinsic emission goes down into the ZnSe substrate 16 and invites the SA-emission (F) by the dopant in the substrate. A part of the SA emission (F) directly goes up. The rest of the SA emission (F) is reflected from the bottom of the substrate, turns upward, and goes out passing through the epitaxial film 17. Mixture of the intrinsic LED light (E) and the SA emission (F) goes out of the LED. The mixture light seems white or neutral colors for eyesight, when the ratio of the LED light and the SA emission is in a pertinent scope. This type takes the normal posture bonding the substrate directly on the top 14 of the stem with the top epitaxial layers like ordinary LEDs. However, weak SA emission is a drawback in this type, because the ratio of the inherent band gap transition emission (E) is more than 50% but the ratio of the SA-emission is less than 50%.
(2) Reverse posture (upside-down) type ZnSe white color LED and neutral color LED
FIG. 5(a) and FIG. 5(b) show an example of a reverse posture type white color LED or neutral color LED of the present invention. FIG. 5(a) is a section of the LED device. FIG. 5(b) is a vertical section of only the LED chip. A transparent plastic mold 21 encloses a stem 22, a stem 23 and an LED chip 25. The xcex93-shaped stem 22 has no cavity on a top branch 24. The top is an even plane. The ZnSe LED 25 is upside down bonded upon the even top 24 of the stem 22. The LED 25 consists of a ZnSe substrate 26 having a dopant as an SA-center and a band gap emission structure 27 (ZnSe-type thin film) epitaxially grown on the substrate 26. The epitaxial film emission structure 27 comprises a ZnSe film, a ZnCdSe film and so on. A pn-junction is formed in the epitaxial laminated films. On the film part, a wide p-electrode has been fabricated. A narrow ring-shaped or a small dot-shaped n-electrode has been made on the bottom of the substrate 26. The LED 25 is turned upside down and is bonded on the top part 24 of the stem 22 at the p-electrode. The top n-electrode is connected to the stem 23 by a wire 28. A single wire is enough for the connection between the chip and the stems. In the reverse posture type, the stem 23 is a cathode and the xcex93-stem 22 is an anode.
Supplying a current from the stem 22 to the stem 23 induces the epitaxial film structure 27 to generate inherent (LED) blue rays (E) of a wavelength of from 460 nm to 510 nm by the electron transition across the band gap. All the blue rays go upward into the substrate 26. Some of the rays further progress out of the substrate as blue light. The rest of the blue rays are absorbed by the SA-centers in the substrate. The SA-center generates yellow SA-rays. The SA-rays also go upward out of the substrate 26. The LED light and the SA emission together emanate upward from the top of the LED. Two different kinds of light mix together. The mixed light seems white or neutral color for eyesight. In the upside down posture, the SA emission increases in proportion to the thickness of the substrate. It is easy to enhance the ratio of the SA-emission more than 50% in the mixture light in the upside-down posture. The reverse posture facilitates the control of the tone of white or neutral colors. However, attention should be paid to the singular relation between the cathode stem 23 and the anode xcex93-shaped stem 22 reverse to the ordinary LEDs.
(3) Encapsulated reverse posture type ZnSe white color LED and ZnSe neutral color LED
The reverse posture type example shown by FIG. 5(a) and FIG. 5(b) has still another drawback. The blue rays emitted from the epitaxial film emission structure 27 nearly in parallel with the surface go out of the side as inherent blue light without mixing with the SA emission. Namely, the side light seems exclusively blue. The shape of the stem (lead) is further now contrived to avoid the side blue light emission. FIG. 6(a) and FIG. 6(b) show an encapsulated reverse posture type of a white color LED or a neutral color LED. FIG. 6(a) is a section of the whole. FIG. 6(b) is a section of only the chip and the neighbor. A transparent plastic mold 31 holds a stem 32, another stem 33 and an LED chip 35 within. The structure is similar to the ordinary LEDs. The xcex93-shaped stem (lead) 32 has a top portion 34 with a deep cavity 39. The ZnSe LED 35 is fixed upside-down upon the bottom of the cavity 39. The depth of the cavity 39 is larger than the height of the LED 35. An upper aperture of the cavity 39 is so narrow that the LED cannot launch rays nearly in parallel to the surface. The cavity 39 forbids the side rays.
The LED 35 includes a ZnSe substrate 36 doped with the dopant atoms which act as SA-emission centers and a (ZnSe-type film) LED emission structure 37 is epitaxially grown on the substrate 36. The LED emission structure 37 includes a ZnSe or ZnCdSe active thin layer and a pn-junction. The LED 35 is fixed upside down upon the bottom 34 of the cavity 39 of the stem 32. The film emission structure 37 has a p-electrode which is directly die-bonded upon the bottom of the stem (lead) 32. The ZnSe substrate 36 has an annular or a small dotted n-electrode on the surface. The n-electrode is connected to the other stem 33 by a wire 38. This type needs only a single wirebonding process. The stem 33 is a cathode and the stem 32 is an anode, since the LED chip is mounted in the reverse posture. An annular reflection plate 40 is mounted on the top of the cavity 39.
When a current flows across the active layer and the pn-junction, the epitaxial film emission structure 37 emits blue rays (E) of 460 nm to 510 nm by the band gap transition. All the blue rays (E) propagate upward and enter the ZnSe substrate 36. The SA-centers built by the dopant atoms absorb a part of the blue rays (E) and generate SA emission (F) ranging from 550 nm to 650 nm in wavelength. The SA-rays (F) also propagate upward together with the rest of the blue rays (E). The blue LED rays (E) mix with the SA-rays (F). The mixture seems white color or neutral color. White color or a neutral color is synthesized by the blue rays (E) and yellow or orange rays (F). All the rays going obliquely from the LED are shielded and reflected by the walls of the cavity 39. Only the rays nearly emitting normal to the chip surface can go out of the cavity 39. This encapsulated upside down type LED has strong directivity.
(4) Reverse posture substrate encapsulating type ZnSe white color LED and ZnSe neutral color LED
The encapsulated type shown by FIG. 6(a) and FIG. 6(b) can cut slantingly-emitting rays. However, this type has a drawback of too strong directivity. Sometimes less directive LEDs are required. Another drawback of the encapsulated type is the complexity of the stem, which raises the cost of producing the complex stem and the cost of mounting the LED on the stem. FIG. 7(a) and FIG. 7(b) show another type of a white color LED or a neutral color LED of the present invention. This type aims at lowering directivity and suppressing the side leakage of blue light. This type gives a cavity to the substrate itself for encapsulating the epitaxial film structure within the substrate.
FIG. 7(a) shows a sectional view of the substrate encapsulating type LED. FIG. 7(b) is a section of the LED chip and the stem. Stems (leads) 42 and 43 and an LED chip 45 are buried in a transparent plastic mold package 41. The notched LED chip 45 is upside down mounted on a top 44 of the xcex93-shaped stem 42. The central part of the LED 45 has a deep cavity 49. A ZnSe-type epitaxial emission structure 47 is formed on the bottom of the cavity 49. The epitaxial emission structure 47 is encapsulated by a substrate 46 itself. All the rays emitted from the epitaxial emission structure 47 must pass through some part of the substrate 46 for going out of the LED device. All the inherent blue rays have chances to be converted to the SA-rays by the ZnSe substrate 46.
There are an insulating layer 50 and an extra ZnSe layer 51 at the peripheral part of the substrate 46. The LED 45 is bonded at the extra ZnSe layer 51 on the stem (lead) surface 44. No electric current flows from the step via the extra ZnSe layer 51 due to the insulating layer 50. A protrusion 52 lies at the center of the stem surface 44. A p-electrode on the epitaxial emission strata is bonded on the protrusion 52. The p-electrode is electrically connected via the protrusion 52 to the stem (lead) 42. The bottom surface of the ZnSe substrate 46 is upside. The substrate 46 has an annular n-electrode or a small dotted n-electrode on the upside. The top n-electrode is connected by a wire 48 to the other stem 43. The stem (lead) 42 is an anode. The other stem 43 is a cathode.
When current is supplied to the LED 45, the epitaxial structure 47 emits shorter wavelength (blue or green) rays (E) between 460 nm and 510 nm. Since the epitaxial emission structure 47 is fully enclosed by the substrate 46, all the rays once pass through the substrate 46. A part of the rays go out as blue or green rays. The rest is absorbed by the substrate 46 and is converted to longer wavelength SA-rays (yellow or orange). Both the blue-green LED-rays (E) and the yellow-orange SA-rays (F) go together in the vertical direction and in the side directions. Mixture of them seems white color or neutral colors for eyesight. This type LED makes low directivity light. Low directivity gives wider applications to this type LED than the former type (3).
Apparently, the white color LED and the neutral color LED of the present invention are not different from conventional LEDs. Every LED of the present invention consists of a substrate and an epitaxial film structure. What is new is doping the substrate with dopants which act as fluorescence centers (or SA-centers). The substrate itself produces fluorescence. The present LEDs dispense with painting or potting an extra fluorescent material (or phosphor) on the LEDs. To spare an extra fluorescent material alleviates material costs, production costs and stem costs. The well-established low-cost manufacturing of conventional LEDs is available. This invention enables the ordinary LED manufacturing technologies to make white color LEDs and neutral color LEDs at low cost.
In any case, a substrate is indispensable for manufacturing an LED. When a substrate emits fluorescence, the fluorescence is deemed to be a hindrance which should be eliminated till now. This invention, however, makes the best use of the fluorescence which is generated from the substrate. Furthermore, this invention accelerates the yield of the fluorescence by doping impurities as origins of the fluorescent emission. This invention succeeds in making white color and neutral colors by adding the fluorescence of the substrate to the band gap emission at the active layer. Conventional LEDs cannot produce the white color and the neutral colors made by the present invention. The success originates from utilizing the obstacle positively.
This invention makes white color light or neutral color light by growing epitaxially ZnSe crystal or ZnSe-related compound on a ZnSe substrate doped with SA-centers, producing blue or bluegreen light by the epitaxial film structure, converting the blue or bluegreen light to yellow or orange light by the SA-centers, and mixing the blue or bluegreen light with the yellow or orange light. This invention has nothing to make white or neutral colors besides an LED chips. The n-type ZnSe this invention relies upon has higher transparency than the YAG phosphor. The higher transparency alleviates the loss of light by absorption. Furthermore, the conversion efficiency of the ZnSe substrate from blue light to yellow or orange light is higher than the YAG fluorescent material. Less absorption and higher conversion give the LEDs of the present invention higher luminosity than the prior GaInN/YAG white LED.
The present invention enjoys a long lifetime due to the ZnSe LED that is a main component of the device. Various tones of white color can be produced by changing the dopants and the dopant concentrations. Further, the white color tone can be varied from warm white to cold white only by changing the thickness of the ZnSe substrate. Unlike the GaInN/YAG white LED, this invention needs no extra fluorescent material. This invention makes the best use of the ZnSe substrate itself as the SA-emission centers. Semiconductor devices, in general, require a substrate for carrying active layers grown thereon. This invention takes advantage of the substrate as a light source of yellow or orange. The exclusion of extra parts from the LED gives this invention a simple structure and facile manufacturing.
This invention first succeeds in making a neutral color LED capable of producing redpurple, pink or bluepurple color which the conventional LEDs never produce. Such neutral colors between red and purple are quite novel for LEDs. This invention has a wide application for display, ornament and lightening.